1. Field of the Invention
The present invention relates to a rotary drum for the treatment, such as drying, heating, cooling, calcining and/or mixing of particulate solids.
2. Prior Art
Rotary drums or kilns are commonly employed, either in a batch process or a continuous process for the treatment, for example, drying and/or calcining, of particulate solids, such as sand, gravel, stone, fertilizers, metal oxides, pigments, various powders and the like. Typically, in a batch process, a drum or kiln containing the material to be treated, is horizontally rotated while heat is applied, either directly or indirectly. In a continuous process, an elongated drum or kiln, inclined slightly from the horizontal, is rotated while the particulate solid to be treated is fed into one end, passes through the rotating drum or kiln and the treated material is discharged at the other end. During passage the particulate solid is contacted with gas, for example, heated or reactive gases, to cause the drying, calcining, or other treatment of the solids. As the drum rotates, the bed of particulate solids is carried or dragged upwardly by friction along the inner surface of the drum until the weight of the particles and the steepness of the slope of the particle bed causes it to slide or tumble. This action continues as the particle bed moves forward toward the discharge end. During the process, efficient gas-solid contact is very important. However, when the solid particles have not been closely screened and the bed consists of a range of particle sizes, there is a tendency for size segregation to occur with the coarse particles forming an upper layer in the moving bed and the finer particles forming a lower layer. The result is an uneven treatment or uneven gas-solid contact and the production of non-uniform product.
It is known to improve the mixing and thus the efficiency of gas-solid contact in such processes through the use of lifting flights attached to the inner wall of the rotating drum. As the drum rotates, the lifting flights serve to lift the particles from the moving bed and then allow them to fall as a curtain or shower back to the particle bed. Although gas-solid contact is improved, the repeated lifting and falling of the particles may result in the production of large amounts of fines and dust which may represent a loss of material and, in addition, may coat the larger particles and thereby interfere with the mixing, drying, and/or calcining or other process being applied. Furthermore, the dust and fines may become entrained in the gas stream, resulting in a potential environmental hazard as the dust laden gases re passed to the atmosphere, or an additional step to remove and/or recover the fines from the exiting gas stream.
In other arts, for example, in the sampling art, the need for efficient mixing has led to the use of various methods and devices. One such device is a riffler. A riffler is a device for mixing material, such as a heterogeneous mass of particulate solids to collect a representative sample. The riffler causes the particulate solids being sampled to be split and recombine multiple times (i.e. over multiple stages) to achieve mixing and obtain a representative sample.
U.S. Pat. No. 3,799,735 discloses the treatment, e.g. calcining, of particulate solids in a rotary kiln equipped with trough-shaped conveyor flights extending helically along the inner wall to discharge the particles over the length of the kiln in the form of clouds of substantially parallel curtains.
U.S. Pat. No. 4,319,888 discloses the use of a riffle plate arrangement to intimately mix or blend a stream of char particles with a stream of coal particles prior to burning.
U.S. Pat. No. 4,136,965 discloses a generally triangular mixer block made from a refractory material and designed for use on the inner wall of a generally horizontal rotating drum. A plurality of the mixer blocks may be used in a kiln to provide a uniform product with minimal formation of fines and dust.
U.S. Pat. No. 5,623,883 discloses a kiln for the calcination of powder wherein the inner circumferential wall is equipped with a plurality of protrusions having the shape of a triangular prism.